Exposure meter



, EXPOSURE METER Filed July 3, 1957 Fi s amen too William E. mum Jr.

Patented Apr. 2,1940

uNi'reusTATes This invention relates to exposuremeters. .One of thechief diiiiculties encountered in photography, especially by'amateurphotographers, is the correct estimation of the'light values presentedby the subject to betphotographedand the selection of the correspondingdiaphragm aperture and the time of exposure with, of course, dueconsideration of the speed of the emulsion which is in use. Unless aphotographer has considerable skill in estimating these values alargeproportion of the resulting pictures will be more 0 or less imperfect. 4H I The production of a perfect .photograph requires a correctcorrelation of the following fac- I tors:

The amount of light illuminating the subject. The predominant wavelengths oi. that light.

The speed or sensitivity of the negative emulsion to be used. As thesensitivity of negative emulsions varies with difierent wave lengths oflight, the speed value must be known for day light, light produced bytungsten filament'incan- 1, descent bulbs, and forany other light whichthe ii pu illary. ,op'eningiin thelhuman eye is itself a j meals re n-f,lightintensity. This fact has been takenjadvantage of by. designers ofexposureme--:

ters. "the petitioner in the'instant application has directed hisinventive thought.

photographer may choose. l

The behavior of the emulsion when used con nection with corrective lensfilters.

The efiect desired. AHfactor not sually considered in exposuremete'rsiis' thefoccasionaldesire of the photographer to produce aphotograph;

which will not be correctly'iexposed from a techthe more prominentdisadvantages of the exposnical standpoint but which will have moreartistic merit if under exposed or over exposeclto some degree.

The above values 'when'duly considered must be correctly evaluated andfrom them a suitable setting for the diaphragm aperture and'exposuretime must be evolved to su'it the"capacity of the h I s provide a directreading exposure meterwith a camera to be used. Numerous attempts havebeen maiie'to assist the photographer in the selection of correctdiaphragm aperture and time of exposure.

Tables or charts prepared forjdifierent light conditions and forvdifferent kinds of plates or films are well known in this art.j'l-Iowever, tables are voluminous and require considerable study andpractice to arrive at the correct result and do not meet the populardemand fora device of this kind.

Another type of device in use obtains aprelim- ,50 inary estimate, oftheprevaLiling light conditions by exposing a sensitized surface to thelightj This type 'of device is not successfulbecause of the considerableamount oftime needed to obtain areading and. the expense'ofv thesensitized paper v needed in itsoperation. t Z p the same:

;. bled device,

Another typeof device works on the principle" of alight sensitive cellwhich produces an'elec trical current varying'in intensity with theamount offilight. Thistype'of device'on the market, while givingfairlyjsatisfactory results, is un- 5 satisfactoryinthatitis' toodelicate and expensive' to, me'et'the demands of the general users of"Still"a'nother type'exposure meter operates on theprinciple or thesensitivity of the human eye 0 itself. ;In this type of device theoperator is called upon, "to view the; object to be photographed-,through"colored glasses of varyingthicknesses.

Thefreading in this type of exposure meter is ,m'ade, by estimating the.point atwhich the out-.15 [lines 'of-theobject can no longer bedistinguished Thisltypejlof meter. and the common variation of thistype'qfjmeter arefvery,unsatisfactory for peoipl'ewho use glassesand'havefTthe addeddisad- "vantage that comp'utation'isn'ece'ssarybefore the 0 camera s etting'van m om the reading" takemw v'ffIt'is a well, t awnaatha' the size ofthe It is this class of exposuremeters that An object of the present invention is to avoid 30 ure metersmentionedfabove and to provide the photographer. with a complete,simple, and co'nvenient instrument which will workequally'well underanylight conditions and with any ommer 35 I cial negative emulsion.

Another object of the present invention is to tures with artisticeffects, such as .under.expos-. ure andover exposure. f-

Other objects and advantages will bapparent during the course of thefollowing description of this invention. r

In the accompanying drawing forming part of this application, and inwhich like numerals are employed to designate like parts throughout Fig.1 represents an edge view pi the assem- Fig. 2 shows the various partsin the same relation as Fig. 1 but disassembled,

Fig. 3 represents a plan of the device as viewed View of the third disk.

The preferred form of this invention is illustrated in the drawingforming a part of this application consists primarily of four aligningdisks provided with indices, with holes in their centers held togetherby a pivotal brad 50 and washer 5|.

The front outside disk I is provided with a small mirror, 2 a drawing ofa human eye3 to one half scale from which the portion representing theiris has been cut out, which will be called the iris opening 4 and anexposure time scale 5 around the outer margin. The second disk In istransparent except where a graduated series of black spots II isprinted. When the second disk-I0 is rotated against the first disk,these black spots II appear one by one through the iris opening 4 ofdisk 'I. A small lug called an emulsion pointer 52 mounted on the hackof the second disk I0 engages in a set of emulsion scale aligningnotches 3! in theth disk 3D. ,The third'disk 20 is proder and overexposure; The pupill'ary opening of the human eye, asiswell known,varies with the intensity of the light entering the eye and is caused bytheau'tomatic vided with'a marginafdiaph'ragm aperture scale 2I. Thisdiaphragm aperture scale 2| when rotated appea s adjacent to theexposure time scale 5 on the front disk I. on the ba'ck'of the thirddisk 20 a correction pointer '22 is provided which is designed to engagewith a set of correction scale aligning notches 32 in the 'fourth'disk30.

The fourth'disk 30, viewed'from the reverseside as has already beenexplained, contains two emulsion scales W and 'S representing Weston andScheine'r readings, a correction scale C to correct for possiblevariation'in the users eye and for corrective filters and to'contro'lthe degree 'of uncontrac-tio'n or dilation of the iris, which tends tomaintain a constant light energy'on the retina. The diameter "of thepupil varies from 2.9 to 8 millimeters while the diameter 'of the irisfor normal humansis about "12 millimetersno matter the age, race, ,orsize of the individual. It follows thata direct measureof the' dilationof the pupillary opening will'repr'esent in inverse pro-' portion anapproximate reading of'the'amount portant'fe'ature's of my presentinvention is the simple direct method provided for measuring the 7diameter of pupillary opening 'by "direct com- 'parisonLi For thispurpose the small mirror 2 is provided on the front of my instrument inwhichthe user may'see an image of his own eye. Such an image viewedinthe mirror -2 Willbe exactly one at normal distances, from about eightinches up Fig. 8 shows a perspective view of the fourth to eighteeninches; from the eye any inaccuracies due to change in the size of thenormal eye will be so negligible as to require no cor-- rection. It isthereforepcssible for the user to hold the mirror at any distance fromthe eye which is convenient to his vision.

The measurement is accomplished by comparing the mirror image of thepupillary opening with the graduated series of black spots on the seconddisk which can be viewe'd singly through the iris opening '4 in thefront disk I. To assist the user in this comparison the illusion isfurthered by a drawing of an eye 3 surrounding this iris opening 4. Inuse an imageof the users eye appears in the mirror and closely adjacentto it can be seen a second eye of equal size in all particulars exceptthe pupil the size of which can be varied by rotating the second diskill so that the various black spots II appear through I the iris opening4. When the black spot I! repof the pupil of the right eye, the left eyemay be directed toward the" object the brightness of which is to bemeasured, and the size of the pupil. of the right eye will beinfluencedby the light'proceeding from the object, "even though the 'ri'gl'i't eye"is obscured by the instrument.

The computing mechanism of this instrument is so 'arra'riged that thenecessary "correction for negative fe 'rnulsion speed, correction fortype of light, for'fi'lters 'iffused, for intentional over or underexposure, and for 'anyvar'iation inthe users eyesight may e pre-se't bysetting the" two' pointers 'I2 fand 22 in the proper IIO'ECHGS'OHthe'reye'r'seside or -the' instrmnent. This setting is to he madeinadvance s iner when the proper black spot I I has been "chosen, thediaphra n aperturereadings Tfand feiiposure time readings 03 will appearn pairso'nfthe front side of'the instrument. Eaen'pan-er figuresrepresents a setting'for the camera which will render a photograph of-eerreet x 'osu're. r

' This lsf'acc o IZfl'l'l nenvais fgwnien provided with the wexosure'nine scale cl and the iris opening 4 through which the b-lack'spots are viewed, does not i otate. The second disk I0 may be rotatedspots II before the: :ri opening '4 one by one. The emulsion pointer '12"on the reverse side of the second disk It engageswith the emulsioninrelatio'n to' the 'firs disk l, placing the "black i p "fs'c'alealigning notches 31 "on the fourth disk of light entering the eye. Oneof the most imcausing 'i 't to rotatejsirnultarlecusly 'with the seconddisk, forfsoflong'as the emulsion pointer "I2 is engaged; 'Bydiseng'aging the emulsion pointer I2 and rotating the fourth disk at inrel'ltlc'ih "t0 the 'S'ec'Ofidi disk I20 "and fre 'enga'ging theemulsion pointer I2 in the proper notch, it"

is pb'ssible'to make the "cdfrectien 'fdr emulsion spe'e'daridtypeof'iignt:

Thefourth. disk 30, as explained above, is

also provided with' correction scale aligning notches 32 which is to beengaged the c'orrectiv'e p'ointer 222w therev'erseside of the third disk21). nieans'o fsca'le c on which is found readings 'indicatm g" overfexposure and under ex- 'posure"aswell as thenormal'setting, it ispossible ishe'd in the following man-' to predetermine the extenflo'f"exposure and to notches.

taking of the picture. v make corrections inthe use of some human eyesScale C isalsowused to that areabnormal. Correction for filters,- etc.,

is made by disengaging the correction pointer:

22 rotating the third disk 20 in relation to the fourth disk 30 and thesecond disk lfl an'dreengaging the correction pointer 22in theproperscale aligningt'notch 32. ll; follows that when I t touched for as longas'the filter is in :use.

the pointers have been set only the first and second disks. may berotated freely-in relation to each other while the second, third,- andfourth disks are forced to'rotate together as asingle unit. I 1 g Thesetting is made by placing the proper black spot ll so it may be viewed.through the iris opening 4 after the various corrections have been madeby setting the pointers in the proper The final corrected reading may'now be read directlyfrom the diaphragm aperture scale ,1, and exposuretime scale d where they appear in pairs.v

In the construction of this deviceany suitable durable material -may beused for making the first disk I, third "disk 20;a'nd fourth disk 30,such as fiber board and the like. The second disk l0 which must betransparent is preferably constructed out of transparent Celluloid orthe like. The pivot 55), washer 5|; and pointers 22 and i2, may be madeout of fiber, metal, or. other suitable material. The mirror 2 may beconstructed out of glass, or bright metal may be used instead accordingto common practice. However, the use of colored mirrors has been foundin manycases is to be used. A convenient list of films may be included nthe back of the instrument if desired. If the emulsion speed of the rollfilm, plate, motion picture film or color process film is not listed onthe back of the instrument, it

will be necessary to obtain this value in Weston or Scheiner units fromthe manufacturer of the taken at night .by artificial light, use thevalue given for tungsten. Of course. the value for tungsten'cannot beused-when the light is furnished by electric arc, neon signs, fires, orother suchsources. This value must be reached by experiment. I I

When the emulsion speed has beendetermined, set the emulsion pointer l2in the emulsion scale aligning notch 3| nearest to the correct figure onthe scale, Scheiner or Weston. The setting is'made by flexing the seconddisk |U,'the Celluloid disk, until the emulsion pointer 12 can berotated to the desirednotch 3|.

When the adjustment for emulsion has been Adjustment for filters If "ahaze filter, color filter, or filter of anyv type is to be used, it ispossibleto pre-set the lowing manner: :22, v:r:3,::z:4, etc.

If pictures are to be taken'in daylight, use the value given fordaylight. If pictures are to be exposure "meter so as to make allowancefor the increased exposure which may be necessary.

This correction is made by setting the correction pointer 22 on the backof the third disk 29 into the proper correction scale aligning notch32.. Filter. factors-are usually given in the fol- This figure isusually found printed on. the filter.

This setting when made, may remain un- When no filter isto be used,leave the'correction pointer 22' at normal, or see exceptions below: A vr Ady'ustmentjor .intemti onal undcr or overexposure The provision hasbeen made, for adjustment for under or over exposurewhere such conditionamount can be determined by experiment and the correction pointer setaccordingly.

How to determine the correct exposure The above instructions have dealtentirely with the adjustment of the scales on the'back of the exposuremeter. The following instructions refer to the front-(mirror side)-'-only:

When the scene tobe photographed has been selected, stand where' thecamera is to be used and look directly at the scene with both.- eyeswide open. Now bring the exposure meter up before your eye about onefoot from the face so that the line of sight of one eye isJintercepted.

.Then with both eyes still wide open, compare in the mirror the size ofthe pupil of your eye with the size of the black spot II in the drawingjust above the mirror. If the sizes of the pupils are different, rotatethe back disks 10, 2B and 383' now locked together by means of pointersl2 and '22 in the respective notches 3| and 32 until the black spot IIshows through the iris opening 4 which is identical with the pupil. a

At this point, the user should be informed of two' considerations.First, the distance which this exposure meter is held from the face willhave only negligible effect on theultimate readtate to do so.

Second, the reading will not be correct if bright lights out-0f range ofthe camera are allowed to "When the black spot H has been selected which,is identical in size to that of the pupil of the users eye, allmanipulation of this exposure ineter has been accomplished and theproper setting for your camera will be foundon the two rotating scaleson the front side, the diaphragm aperture scale ,5andthe timeexposurescale 2| (see Fig. 3 of the drawing). a

With the instrument set as described above the last step is to select asuitable exposure time from the exposure time scale '5 and the correctcorresponding aperture opening on the diaphragm aperture scale 2i willbe: found alongside the first reading.

Having thus described the invention, I claim: 1; An exposure meter foruse in, photography comprising a front plate, provided with a, timeexposure scale on its front face, and provided with a mirror on itsfront face adapted to reflect an image of the pupil of a human eye, saidfront plate being also providedwith a hole immediately adjacent to thesaid mirrorwthroughwhich a spot simulating the pupil .of the human eyemay be viewed combined in a slidable contacting relationship with asecond plate which is provided with a series of spots simulating thepupil of theeye in itsvarying sizes due-t0 light intensities, the saidspots being arranged on thesaid second plate so as to consecutively showthrough the said openings of the said front plate for comparison withthe reflected image of the pupil of the human eye in the said mirror onthe said front plate, and a third plate provided with a diaphragmaperture scale combined in-a slida-ble and contacting relationship withthe said second plate, thesaiddiaphragm aperture scale of the said thirdplate being arranged to align with the said time exposure scale of thesaid front plate, the said second plate and the said third plate beingprovided with locking means whereby a relative movement of the secondplate with the front plate during the comparison of the spots on thesecond plate with thereflec-ted image of the eyein'the: mirror. on the,front plate causes theflthird plate to move with respect to the frontplate and align its; diaphragm opening scale, with 2. The-device asclaimed in claim 1, wherein the saidhlocking means for locking the saidsecv0nd plate. to the said third plate comprises a fourthplate providedwith a series of notches in its edge. adapted toreceive a lug extendingfrom the back side of the second plate, said fourth plate being providedwith a correction scale for the possible variation in the users eyes,filter, or degree of under or over exposure aligned with the series ofnotches whereby the selection of one of, the notches brings about acorrection setting for, the possible variation. in the users eyes,filter, or degree of under or over exposure.

3. The device as claimed inclaim 1 wherein the said means for lockingthe said second plate to the said, third plate comprises a fourth platehaving a series of notches in its edge adapted to receivea lug extendingfrom-the back side of the said thirdplate-said fourthplate beingprovided with a film emulsion correction scale aligned with the seriesof notches whereby the setting of the said lug in a notch constitutes acorrection setting for a films emulsion variation.

WILLIAM E. WALLACEQJR.

